Angelo, this was discussed, again, just a few days ago. It was not, as Amaral has asserted, just a matter of a stamp on a letter.
G-Unit dug out the relevant passage:
Nevertheless, anyone who feels unsatisfied about the epilogue of the investigations, will have the possibility to react against it, having the possibility of eventually changing that epilogue, by prompting diligences based on new evidence, as long as that person has the legitimacy to request them and the requested diligences are serious, pertinent and consequent. They may do so in three ways: by requesting the reopening of the inquiry, under article 279, number 1 of the Penal Process Code; by appealing hierarchically against this dispatch under number 2 of article 278, or in another case, under number 2 of article 279 of the Penal Process Code, or by requesting the opening of the instruction under article 287, number 1, item b, of the Penal Process Code.
I have no cite for this, but as I understand it;
Once the archiving was announced, the arguidos had 20 days to ask for the case to move to the 'instruction' phase. This involves an 'instruction judge' holding an autonomous investigation and an instruction debate between all the parties. At the end of this the judge decides whether a trial should take place or not.
Asking for this would have given the McCanns' lawyer a chance to put their arguments to a judge. Had he decided not to proceed to trial it would have helped to allay suspicions, I would think.
Even after the 20 day deadline offering to take part in the reconstruction or to answer the questions Kate declined to answer would have sufficed.